The present invention relates to a method and device for measuring resistance and is particularly applicable to an electronic thermometer employing a thermistor as a temperature sensitive element. Such a thermometer may be used for measuring body temperature of a human or animal subject or for measuring ambient temperature.
It is known to utilize a temperature-dependent resistive element together with an oscillator circuit to form a digital thermometer. In the prior art, a thermistor is sometimes used as a temperature-dependent, variable-resistance element in series with a charging capacitor to form the frequency-controlling elements of the oscillator network. The equation
  f  =      1          2      ⁢      π      ⁢                          ⁢      RC      determines the frequency of oscillation, where R is the resistance of the resistive element (thermistor) and C is the capacitance of the series charging capacitor. As the temperature varies, the resistance of the thermistor varies, and the frequency varies as a result. By measuring the frequency, and knowing the value of the capacitance, the value of R can be determined. Because R is uniquely related to temperature, the temperature can be determined as well. For a thermistor, the resistance is related to the temperature via the Steinhart-Hart equation. The use of a multivibrator as the oscillator circuit is disclosed in U.S. Pat. No. 4,359,285 by Washburn for low-power oceanographic applications. U.S. Pat. Nos. 4,602,871 and 4,464,067 issued to Hanaoka disclose thermometers based on thermistor-controlled oscillators whose properties emphasize miniaturization, light weight, and improved accuracy using correcting circuits. These latter two patents refer to applications wherein the sensor may be used with low-power wristwatch devices.
One disadvantage of measuring the frequency of the oscillator is that one must know the value of the capacitor extremely accurately in order to derive the value of the resistance accurately. Generally, it is difficult to do capacitance measurements accurately, and in addition, the capacitance value is known to be a temperature-dependent parameter. The capacitance can increase or decrease with changing temperature and the degree of change is related to the exact type of material used in the capacitor (Y5V, X7R, NPO, etc.). A further disadvantage of this approach is that the active circuit elements in the oscillator circuit can themselves have temperature-dependencies. These dependencies are nearly impossible to predict and may vary from circuit to circuit.
Some attempts have been made to reduce the undesirable temperature dependencies by way of calibration techniques. As an example of prior art, U.S. Pat. No. 4,150,573 discloses the use of a thermistor to control a pulse oscillator circuit. In that patent, the pulse oscillator input is switched between the thermistor and a fixed resistor. A ratio is formed between the frequency produced by the thermistor and the frequency produced by the fixed resistor. This ratio divides out uncertainties associated with circuit component values and power supply variations. This provides the advantage of reducing the need for high accuracy parts and reduces the effects of power supply variations. However, this approach is unnecessarily complicated and it does not accurately measure the non-ideal behavior of the oscillator circuit nor does it null out temperature dependencies in the active components of the oscillator circuit. This approach may also introduce errors due to the temperature variations in the switching device.
For a medical thermometer, or other applications where extreme accuracy is required (less than 0.25 degrees C. uncertainty), the errors that are introduced by capacitance variation and by active circuit element variation must be minimized. In addition, for a low-power application such as an ingestible temperature sensor, it is not possible to use sophisticated, computer-controlled correction techniques, because the thermometer must be miniature, and is expected to be powered from a 1.5 volt battery source or a 3.0 volt battery source.
At present there are ingestible temperature responsive transmitters or ingestible temperature monitoring pills available. U.S. Pat. No. 4,689,621 issued to Kleinberg, and U.S. Pat. No. 4,844,076 issued to Lesho et al describe temperature responsive transmitters for use in ingestible capsules. Both devices disclosed employ crystal-controlled oscillators which transmit continuously on a single frequency determined by the temperature of the device. Lesho et al. also discloses a receiver employing a frequency counter to determine the frequency of the transmitter, and perform the calculation to determine the temperature sensed by the pill. Limitations of these prior art ingestible thermometer designs have been articulated in U.S. Pat. No. 6,629,776 (Bell, et al.), in which a highly accurate method of measuring temperature is disclosed.
A known technique for measuring the ratio of an unknown voltage to a known voltage employs the so-called sigma-delta converter. A sigma-delta converter consists of two essential functional parts: an integrator-summer (Σ) and a comparator (Δ). In a conventional Σ-Δ ADC, a comparator compares an unknown voltage and a voltage generated by a digital pulse and integrated through a lowpass filter. The output of the comparator is latched into a D-flip flop and the inverted output of the D-flip flop forms the next digital pulse that is provided to the lowpass filter. If the output of the comparator is a logic 1 (high), the output pulse is negative rail. If the output of the comparator is a logic 0 (low), the output pulse is positive rail. Inverting the output of the D-flip flop provides negative feedback. The digital pulses are integrated by the lowpass filter resulting in a steady-state voltage which is equal to the unknown voltage. A fixed number of pulses is generated, and the number of 1's counted. The ratio of the number of 1's to the total number of pulses is the same as the ratio of the unknown voltage to the total voltage swing (positive minus negative rail voltages).